This invention relates to two-cycle internal combustion engines intended in particular to equip motor vehicles and it relates in particular to the circuit for exhaust of used combustion gases to be mounted on such engines.
The internal combustion engines most often used to equip the vehicles operate according to a four cycle: intake by a valve of the air-fuel mixture, followed by its compression, then by its expansion after combustion and finally exhaust of the burnt gases by a second valve.
In a two-cycle engine, the piston rises toward a top dead center position compressing the gases in the cylinder. Then, the combustion takes place and the released energy is recovered in the downward movement of the piston. The intake and exhaust times are generally controlled by the piston which moves past the intake and exhaust ports, located in the body of the cylinder near the bottom dead center: first, the exhaust port making it possible for the burnt gases to begin to escape, then the intake port feeding the cylinder pressurized "fresh air". These fresh gases are used to scavenge the burnt gases toward the exhaust. As a result, a part of the fresh gases are lost to the exhaust when the fuel is simultaneously introduced to the fresh gases, more than 25% from the fuel introduced in the combustion chamber does not burn and is directly rejected with the exhaust gases into the atmosphere. An extensive pollution results from this, on the order of ten to twenty times greater than that of a four-cycle engine.
New techniques have been developed to respond to the problem of fuel losses by exhaust. Instead of jointly introducing the fuel and combustion fresh air, only the latter scavenges the combustion gases.
The introduction of the fuel is delayed and is performed when the exhaust port is nearly closed to minimize losses from it. Various injection devices have been developed. The applicant has particularly studied a direct injection device with stratified charge making possible a great reduction of the emissions of unburned residues as well as nitrogen oxides.
The natural pollution control produced by the injection of the fuel after closing the exhaust port is still not sufficient to meet the increasingly draconian antipollution laws.
It is therefore necessary to combine catalytic converters with the two-cycle engines.
Unfortunately, the design of two-cycle engines prevents fully using all the pollution control possibilities of the catalytic converters.
Actually, the burnt gases are evacuated mixed with fresh scavenging air, it is therefore impossible to work in a reducing medium and to process the nitrogen oxides or NO.sub.x.